The most advanced techniques of long-term blood cell preservation utilize freezing of the blood at cryogenic temperatures, for example around -195.degree. C. by the use of liquid nitrogen, dry ice, or the like. A review of such techniques is available in an article entitled "Frozen Blood: Principles of Practical Preservation", p.p. 133-173, in Monographs in the Surgical Sciences Volume 3, No. 3, (Williams and Wilkins Co., 1956).
Preferably, the blood cells are stored in a flexible, collapsible container made of a blood-compatible plastic material. This permits the collapse of the bag for complete removal of the blood cells from the container with a minimum of suspending solution after freezing.
Currently, a blood cell freezing bag is sold by the Union Carbide Company, made of biaxially-oriented polyethylene with a wall thickness of about 0.002 to 0.004 inch. This container bag functions adequately at room temperature, when the blood cells are being placed into it or removed from it. However, it has exhibited an undue amount of weakness at cryogenic temperatures of about -195.degree. C., so that upon rough handling the brittle plastic at this low temperature sometimes breaks. This of course usually spoils the blood cells, and prevents their use.
Furthermore, the polyethylene bag cannot be made by the desirable radio frequency (R.F.) sealing techniques for sealing the edges and ends of the bag together to provide a hermetically sealed container. Polyethylene is not effectively responsive to R.F. sealing, so less desirable heat sealing techniques must be used in the manufacture of the container, resulting in weaker seals.
While it would of course be desirable to increase the wall thickness of polyethylene freezing containers to approximately 0.008 to 0.01 inch, in order to increase the low temperature impact strength of the containers, this expedient is impractical, because such containers are excessively stiff, and do not collapse easily to permit the efficient removal of the blood cells from the bag.
Accordingly, the manufacturers of polyethylene cell freezing bags have been forced to accept a product which is undesirably fragile at low temperatures.
The blood cell freezing container of this invention provides a flexible, collapsible bag which has greatly increased impact strength at cryogenic temperatures, while retaining the good flexibility and collapsibility at room temperature for convenient processing of the blood cells. Furthermore, the collapsible container of this invention can be manufactured by the use of a convenient R.F. sealing process, as is conventionally used in the manufacture of polyvinyl chloride blood bags on a commercial basis, to produce the best available seal.
Accordingly, this superior product can stand rougher handling at cryogenic temperatures, while being capable of being effectively mass produced with R.F. sealing techniques.